The generation and recording of borehole acoustic waves is a key measurement employed in oilfield wellbore logging. Many wellbore tools and methods are currently available for taking acoustic measurements. Some tools include a single source of acoustic energy and two or more receivers; however, most of the tools now include many receivers arranged in an array. While the currently available acoustic tools are useful in providing a large range of information regarding the adjacent formation and the borehole parameters, a primary use of acoustic borehole measurements is the estimation of formation slowness. Usually the measurements are taken in the sonic domain, with frequencies typically in the range of 500 Hz to 25 kHz.
Compressional or extensional wave formation slowness is typically estimated using travel times acquired via a motion detection process. In the case of a single source, two receiver tool suggested by the prior art, formation slowness is estimated by subtracting the arrival times between two receivers and dividing by the inter-receiver spacing. This estimate, however, is subject to inaccuracies due to tool tilt, borehole washouts, bed boundary effects, etc. Additional acoustic sources and receivers and more robust methods such as STC (Slowness-Time-Coherency analysis) among others have been used to reduce the inaccuracies introduced by such environmental effects.
One example of a sonic tool according the prior art is shown in FIG. 1. FIG. 1 illustrates a Dipole Sonic Imaging (DSI) tool (100). The DSI tool (100) includes a processing and telemetry cartridge (102), a sonic receiver section (104), and a sonic transmitter section (106). The processing and telemetry cartridge (102) may include a computer processor for controlling the firing of sonic transmitters from the sonic transmitter section (106), the receipt of waveform measurements by the sonic receiver section (104), and communication to uphole controls and equipment.
As shown in FIG. 1, the sonic receiver section (104) includes an array of individual sonic receivers (108). The sonic transmitter section (106) includes a monopole transmitter (110), and upper and lower dipole transmitters (112, 114), respectively. The monopole transmitter (110) and the upper and lower dipole transmitters (112, 114), as well as the sonic receivers (108), facilitate compressional and shear measurements through adjacent formations. The tool (100) may operate in several data acquisition modes to acquire different waveforms. The modes may include upper and lower dipole modes, crossed dipole mode, Stoneley mode, P and S wave modes, and first motion mode.
However, a common problem encountered with sonic logging is the propagation of generated signals along the tool (100) itself. The signals propagating along the tool (100) are commonly known as a “tool arrivals” and are considered “noise” that can interfere with the detection of signals corresponding to the formation. Therefore, a number of approaches have been taken to remove or reduce tool arrivals. The most common approach to reducing the effects of tool arrivals is to insert an isolator between the transmitter section (106) and the receiver section (104) as shown in FIG. 1. The intent of the isolator is to prevent, attenuate, and/or delay propagation of the tool arrival. According to FIG. 1, the isolator is an isolation joint (116), further described in U.S. Pat. Nos. 4,872,825 and 5,036,945.
Additionally, during sonic logging there is a recoupling of signals from the borehole into the tool (100). Because of this recoupling, it is helpful to design a slow and/or highly damped receiver section so that the recoupled tool signals do not interfere with the formation signals. According to FIG. 1, the receiver section (104) includes a slotted sleeve (111) that functions as a slow structural member. The slotted sleeve (111) is further described in U.S. Pat. Nos. 4,850,450 and 6,494,288. The slotted sleeve (111) usually provides the mechanical strength necessary for sonic logging operations and reduces tool arrivals. However, it is very difficult or impossible to use the slotted sleeve (111) and measure formation slowness greater than about 700 μs/ft, especially while maintaining sufficient mechanical strength. Further, certain borehole modes, such as Stoneley and leaky compressional (P) modes, are not efficiently excited because of the non-smooth surface of the slotted sleeve (111).
The slotted sleeve (111) also typically houses and mechanically protects the individual sonic receivers (108) and associated electronics. And although the slotted sleeve housings provide acoustic delays between the transmitter and receiver elements, they simultaneously provide detrimental acoustic reflections and other undesirable secondary acoustic energy or noise in the vicinity of the receivers.
The undesirable secondary acoustic energy, or noise, is developed primarily due to the discontinuous pattern formed in the housing section surrounding the receivers. Sound waves traveling along the tortuous path of the slotted sleeve housings develop reflected scatter patterns when interfacing with the change in pattern from the discontinuous slots to the open rectangular windows about or in the vicinity of the receivers. Additional noise is developed in the prior housings due to the “ringing” of the short cylindrical elements that make up the slotted sleeve. The noise produced inherently by the design of the prior slotted sleeve housings limits their effectiveness, especially in full wave form logging operations in wide frequency band.
Because none of the prior approaches has been completely successful in removing interfering signals and providing adequate mechanical strength, the present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems outlined above.